Transcript Chromosome Lecture - Mayfield City Schools

Overview: Locating Genes Along Chromosomes
 Mendel’s “hereditary factors” were genes
 Today we know that genes are located on
chromosomes
 The location of a particular gene can be seen by
tagging isolated chromosomes with a fluorescent
dye that highlights the gene
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Figure 12.1
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Concept 12.1: Mendelian inheritance has its physical
basis in the behavior of chromosomes
 Mitosis and meiosis were first described in the late
1800s
 The chromosome theory of inheritance states
 Mendelian genes have specific loci (positions) on
chromosomes
 Chromosomes undergo segregation and independent
assortment
 The behavior of chromosomes during meiosis can
account for Mendel’s laws of segregation and
independent assortment
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1/13
 Some Mendel
 Some Chromosomal Inheritance
 Where are Mendel’s alleles?
 Barr Bodies
 Linked Genes
 Lets review!
 Things to Study: Nondisjunctions
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Figure 12.2
P Generation
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Y
R
Y
r
R
y
r
y
Meiosis
Fertilization
Gametes
r
y
R Y
All F1 plants produce
yellow-round seeds (YyRr).
F1 Generation
R
R
y
r
y
r
Y
Y
Meiosis
LAW OF SEGREGATION
The two alleles for each
gene separate.
R
r
Y
y
r
R
Y
y
Metaphase
I
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently.
1
1
R
r
Y
y
r
R
Y
y
Anaphase I
R
r
Y
y
Metaphase
II
r
R
Y
y
2
2
R
R
/4
1
YR
F2 Generation
3 Fertilization
recombines the R and
r alleles at random.
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y
Y
Y
r
r
r
/4
1
yr
Y
Y
y
YR
r
/4
1
Yr
An F1  F1 cross-fertilization
9
:3
:3
:1
y
y
R
R
/4
1
yR
3 Fertilization results in the
9:3:3:1 phenotypic ratio
in the F2 generation.
Figure 12.2a
P Generation
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Y
Y
r
R R
y
y
r
Meiosis
Fertilization
Gametes
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R Y
y
r
Figure 12.2b
F1 Generation
R
All F1 plants produce
yellow-round seeds (YyRr).
R
y
r
y
r
Y
Y
LAW OF INDEPENDENT
ASSORTMENT
Alleles of genes on
nonhomologous
chromosomes assort
independently.
Meiosis
LAW OF
SEGREGATION
The two alleles for
each gene separate.
R
r
Y
y
r
R
Y
y
Metaphase
I
1
1
R
r
r
R
Y
y
Anaphase I
Y
y
r
R
Metaphase
II
R
r
2
2
y
Y
Y
R
R

14
YR
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r

14
yr
Y
Y
y
r
y
Y
y
Y
r
r

14
Yr
y
y
R
R

14
yR
Figure 12.2c
LAW OF
SEGREGATION
LAW OF
INDEPENDENT
ASSORTMENT
F2 Generation
3 Fertilization
An F1  F1 cross-fertilization 3 Fertilization results
in the 9:3:3:1
recombines the
phenotypic ratio in
R and r alleles
9
:3
:3
:1
the F2 generation.
at random.
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Morgan’s Experimental Evidence: Scientific
Inquiry
 Thomas Hunt Morgan and his students began
studying the genetics of the fruit fly, Drosophila
melanogaster, in 1907
 Several characteristics make fruit flies a convenient
organism for genetic studies
 They produce many offspring
 A generation can be bred every two weeks
 They have only four pairs of chromosomes
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Figure 12.3
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Figure 12.3a
mutant (part 1: wild-type)
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Figure 12.3b
(part 2: mutant)
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Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
 In one experiment, Morgan mated male flies with
white eyes (mutant) with female flies with red eyes
(wild type)
 The F1 generation all had red eyes
 The F2 generation showed the classical 3:1 red:white
ratio, but only males had white eyes
 Morgan concluded that the eye color was related to
the sex of the fly
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 Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
 Morgan’s finding supported the chromosome theory
of inheritance
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Figure 12.4
Experiment
P
Generation
F1
Generation
Conclusion
P
Generation
X
X
w
w
All offspring
had red eyes.
w
Eggs
Results
F2
Generation
X
Y
F1
Generation
w
Sperm
w
w
w
w
Eggs
F2
Generation
w
w
Sperm
w
w
w
w
w
w
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w
Figure 12.4a
Experiment
P
Generation
F1
Generation
Results
F2
Generation
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All offspring
had red eyes.
Figure 12.4b
Conclusion
P
Generation
X
X
w
X
Y
w
w
Eggs
F1
Generation
Sperm
w
w
w
w
w
Eggs
F2
Generation
w
w
Sperm
w
w
w
w
w
w
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w
Concept 12.2: Sex-linked genes exhibit unique
patterns of inheritance
 In humans and some other animals, there is a
chromosomal basis of sex determination
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The Chromosomal Basis of Sex
 In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
 Only the ends of the Y chromosome have regions
that are homologous with corresponding regions of
the X chromosome
 The SRY gene on the Y chromosome is required for
the developments of testes
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Figure 12.5
X
Y
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 Females are XX, and males are XY
 Each ovum contains an X chromosome, while a
sperm may contain either an X or a Y chromosome
 Other animals have different methods of sex
determination
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Figure 12.6
44 
XY
Parents
22 
X
22 
22 
or Y
X
Sperm
44 
XX
44 
XX
Egg
or
44 
XY
Zygotes (offspring)
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 A gene that is located on either sex chromosome is
called a sex-linked gene
 Genes on the Y chromosome are called Y-linked
genes; there are few of these
 Genes on the X chromosome are called X-linked
genes
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Inheritance of X-Linked Genes
 X chromosomes have genes for many characters
unrelated to sex, whereas the Y chromosome
mainly encodes genes related to sex determination
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 X-linked genes follow specific patterns of inheritance
 For a recessive X-linked trait to be expressed
 A female needs two copies of the allele
(homozygous)
 A male needs only one copy of the allele
(hemizygous)
 X-linked recessive disorders are much more
common in males than in females
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Figure 12.7
X NX N
Xn
XN
Sperm
Y
Eggs XN
X NX n X NY
XN
X NX n X NY
(a)
X NX n
XnY
X NX n
X NY
Y
Sperm
Xn
XnY
Y
Eggs XN
X NX N X NY
Eggs XN
XNXn XNY
Xn
XNXn XnY
Xn
XnXn XnY
(b)
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(c)
Sperm
 Some disorders caused by recessive alleles on the
X chromosome in humans
 Color blindness (mostly X-linked)
 Duchenne muscular dystrophy
 Hemophilia
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X Inactivation in Female Mammals
 In mammalian females, one of the two X
chromosomes in each cell is randomly inactivated
during embryonic development
 The inactive X condenses into a Barr body
 If a female is heterozygous for a particular gene
located on the X chromosome, she will be a mosaic
for that character
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Figure 12.8
X chromosomes
Early embryo:
Two cell
populations
in adult cat:
Allele for
black fur
Cell division and
X chromosome
inactivation
Active X
Inactive
X
Active X
Black fur
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Allele for
orange fur
Orange fur
Figure 12.8a
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Concept 12.3: Linked genes tend to be inherited
together because they are located near each other
on the same chromosome
 Each chromosome has hundreds or thousands of
genes (except the Y chromosome)
 Genes located on the same chromosome that tend
to be inherited together are called linked genes
http://learn.genetics.utah.edu/co
ntent/pigeons/geneticlinkage/
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How Linkage Affects Inheritance
 Morgan did experiments with fruit flies that show
how linkage affects inheritance of two characters
 Morgan crossed flies that differed in traits of body
color and wing size
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 Morgan found that body color and wing size are
usually inherited together in specific combinations
(parental phenotypes)
 He reasoned that since these genes did not assort
independently, they were on the same chromosome
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Figure 12.UN01
F1 dihybrid female
and homozygous
recessive male
in testcross
b vg
b vg
b vg
b vg
b vg
b vg
Most offspring
or
b vg
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b vg
Figure 12.9
Experiment
P Generation
(homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b b vg vg
F1 dihybrid testcross
Homozygous
recessive (black
body, vestigial
wings)
Wild-type F1 dihybrid
(gray body, normal wings)
b b vg vg
b b vg vg
Testcross
offspring
b vg
b vg
b vg
Wild-type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b b vg vg
b b vg vg
Eggs b vg
b vg
Sperm
b b vg vg b b vg vg
PREDICTED RATIOS
Genes on different
chromosomes:
1
:
1
:
1
:
1
Genes on some
chromosome:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
Results
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Figure 12.9a
Experiment
P Generation
(homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b b vg vg
F1 dihybrid testcross
Wild-type F1 dihybrid
(gray body, normal wings)
b b vg vg
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Homozygous
recessive (black
body, vestigial
wings)
b b vg vg
Figure 12.9b
Experiment
Testcross
offspring
Eggs b vg
b vg
b vg
b vg
Grayvestigial
Wild-type
Black(gray-normal) vestigial
Blacknormal
b vg
Sperm
b b vg vg
b b vg vg
b b vg vg b b vg vg
PREDICTED RATIOS
Genes on different
chromosomes:
1
:
1
:
1
:
1
Genes on same
chromosome:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
Results
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 However, nonparental phenotypes were also
produced
 Understanding this result involves exploring genetic
recombination, the production of offspring with
combinations of traits differing from either parent
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Genetic Recombination and Linkage
 The genetic findings of Mendel and Morgan relate to
the chromosomal basis of recombination
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Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
 Mendel observed that combinations of traits in some
offspring differ from either parent
 Offspring with a phenotype matching one of the
parental phenotypes are called parental types
 Offspring with nonparental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
 A 50% frequency of recombination is observed for
any two genes on different chromosomes
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Figure 12.UN02
Gametes from yellow-round
dihybrid parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
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Recombinant
offspring
Recombination of Linked Genes: Crossing Over
 Morgan discovered that even when two genes were
on the same chromosome, some recombinant
phenotypes were observed
 He proposed that some process must occasionally
break the physical connection between genes on
the same chromosome
 That mechanism was the crossing over between
homologous chromosomes
Animation: Crossing Over
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Figure 12.10
P generation
(homozygous)
Wild type (gray body,
normal wings)
F1 dihybrid testcross
Double mutant (black body,
vestigial wings)
b vg+
b vg
b vg+
b vg
Wild-type F1 dihybrid
(gray body, normal wings)
Homozygous recessive
(black body, vestigial wings)
b vg+
b vg
b vg
b vg
Replication
of chromosomes
Meiosis I
Replication
of chromosomes
b vg+
b vg
b vg+
b vg
b vg
b vg
b vg
b vg
b vg+
Meiosis I and II
b vg
b vg
b vg
Meiosis II
b vg+
b vg
b vg
944
Blackvestigial
206
Grayvestigial
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
b vg
185
Blacknormal
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Parental-type offspring
Recombination

frequency
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Recombinant
chromosomes
Recombinant offspring
391 recombinants
2,300 total offspring
 100  17%
b vg
Sperm
Figure 12.10a
P generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b vg+
b vg
b vg+
b vg
Wild-type F1 dihybrid
(gray body,
normal wings)
b vg+
b vg
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Figure 12.10b
F1 dihybrid testcross
Wild-type F1
dihybrid
(gray body,
normal wings)
b vg+
b vg
b vg
Meiosis I
b vg
b vg+
b vg
b vg+
b vg
b vg
b vg
b vg
b vg
Homozygous
recessive
(black body,
vestigial wings)
b vg+
Meiosis I and II
b
vg
b vg
b vg
Meiosis II
Eggs
b+ vg+
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b vg
Recombinant
chromosomes
b+ vg
b vg+
b vg
Sperm
Figure 12.10c
Recombinant
chromosomes
b vg+
b vg
b vg
944
Blackvestigial
206
Grayvestigial
b vg
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
185
Blacknormal
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Parental-type offspring
Recombinant offspring
Recombination
391 recombinants  100  17%

frequency
2,300 total offspring
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b vg
Sperm
New Combinations of Alleles: Variation for
Normal Selection
 Recombinant chromosomes bring alleles together in
new combinations in gametes
 Random fertilization increases even further the
number of variant combinations that can be
produced
 This abundance of genetic variation is the raw
material upon which natural selection works
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Mapping the Distance Between Genes Using
Recombination Data: Scientific Inquiry
 Alfred Sturtevant, one of Morgan’s students,
constructed a genetic map, an ordered list of the
genetic loci along a particular chromosome
 Sturtevant predicted that the farther apart two genes
are, the higher the probability that a crossover will
occur between them and therefore the higher the
recombination frequency
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 A linkage map is a genetic map of a chromosome
based on recombination frequencies
 Distances between genes can be expressed as map
units; one map unit represents a 1% recombination
frequency
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Figure 12.11
Results
Recombination
frequencies
9%
Chromosome
17%
b
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9.5%
cn
vg
 Genes that are far apart on the same chromosome
can have a recombination frequency near 50%
 Such genes are physically linked, but genetically
unlinked, and behave as if found on different
chromosomes
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 Sturtevant used recombination frequencies to make
linkage maps of fruit fly genes
 Using methods like chromosomal banding,
geneticists can develop cytogenetic maps of
chromosomes
 Cytogenetic maps indicate the positions of genes
with respect to chromosomal features
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Figure 12.12
Mutant phenotypes
Short
aristae
0
Long aristae
(appendages
on head)
Black
body
Cinnabar
eyes
48.5 57.5
Gray
body
Red
eyes
Vestigial
wings
67.0
Normal
wings
Wild-type phenotypes
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Brown
eyes
104.5
Red
eyes
Concept 12.4: Alterations of chromosome number
or structure cause some genetic disorders
 Large-scale chromosomal alterations in humans
and other mammals often lead to spontaneous
abortions (miscarriages) or cause a variety of
developmental disorders
 Plants tolerate such genetic changes better than
animals do
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Abnormal Chromosome Number
 In nondisjunction, pairs of homologous
chromosomes do not separate normally
during meiosis
 As a result, one gamete receives two of the same
type of chromosome, and another gamete receives
no copy
Video: Nondisjunction
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Figure 12.13-1
Meiosis I
Nondisjunction
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Figure 12.13-2
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
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Figure 12.13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n1
n1
n−1
n−1
n1
n−1
n
n
Number of chromosomes
(a) Nondisjunction of homologous chromosomes in
meiosis I
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(b) Nondisjunction of sister
chromatids in meiosis II
 Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
 Offspring with this condition have an abnormal
number of a particular chromosome
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 A monosomic zygote has only one copy of a
particular chromosome
 A trisomic zygote has three copies of a particular
chromosome
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 Polyploidy is a condition in which an organism has
more than two complete sets of chromosomes
 Triploidy (3n) is three sets of chromosomes
 Tetraploidy (4n) is four sets of chromosomes
 Polyploidy is common in plants, but not animals
 Polyploids are more normal in appearance than
aneuploids
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Alterations of Chromosome Structure
 Breakage of a chromosome can lead to four types of
changes in chromosome structure
 Deletion removes a chromosomal segment
 Duplication repeats a segment
 Inversion reverses orientation of a segment within a
chromosome
 Translocation moves a segment from one
chromosome to another
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Figure 12.14
(a) Deletion
(c) Inversion
A deletion removes a
chromosomal segment.
(b) Duplication
An inversion reverses a segment
within a chromosome.
(d) Translocation
A duplication repeats
a segment.
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A translocation moves a segment
from one chromosome to a
nonhomologous chromosome.
Figure 12.14a
(a) Deletion
A deletion removes a
chromosomal segment.
(b) Duplication
A duplication repeats
a segment.
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Figure 12.14b
(c) Inversion
An inversion reverses a segment
within a chromosome.
(d) Translocation
A translocation moves a segment
from one chromosome to a
nonhomologous chromosome.
© 2014 Pearson Education, Inc.
 A diploid embryo that is homozygous for a large
deletion is likely missing a number of essential
genes; such a condition is generally lethal
 Duplications and translocations also tend to be
harmful
 In inversions, the balance of genes is normal but
phenotype may be influenced if the expression of
genes is altered
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Human Disorders Due to Chromosomal
Alterations
 Alterations of chromosome number and structure
are associated with some serious disorders
 Some types of aneuploidy upset the genetic
balance less than others, resulting in individuals
surviving to birth and beyond
 These surviving individuals have a set of
symptoms, or syndrome, characteristic of the type
of aneuploidy
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Down Syndrome (Trisomy 21)
 Down syndrome is an aneuploid condition that
results from three copies of chromosome 21
 It affects about one out of every 700 children born in
the United States
 The frequency of Down syndrome increases with the
age of the mother, a correlation that has not been
explained
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Figure 12.15
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Figure 12.15a
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Figure 12.15b
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Aneuploidy of Sex Chromosomes
 Nondisjunction of sex chromosomes produces a
variety of aneuploid conditions
 Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY individuals
 Females with trisomy X (XXX) have no unusual
physical features except being slightly taller than
average
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 Monosomy X, called Turner syndrome, produces
X0 females, who are sterile
 It is the only known viable monosomy in humans
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Disorders Caused by Structurally Altered
Chromosomes
 The syndrome cri du chat (“cry of the cat”) results
from a specific deletion in chromosome 5
 A child born with this syndrome is mentally retarded
and has a catlike cry; individuals usually die in
infancy or early childhood
 Certain cancers, including chronic myelogenous
leukemia (CML), are caused by translocations of
chromosomes
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Figure 12.16
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
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Figure 12.UN03a
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Figure 12.UN03b
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Figure 12.UN04
Sperm
P generation
gametes
D
C
B
A
d
E
F
D
e
C
B
A
F
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Egg
e
f
This F1 cell has 2n  6 chromosomes and is heterozygous
for all six genes shown
(AaBbCcDdEeFf).
Red  maternal; blue  paternal.
Each chromosome has
hundreds or thousands
of genes. Four (A, B, C,
F) are shown on this one.
c
b
a
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e)
or so far apart on the
same chromosome
(c and f) that they
assort independently.
d
E
cb
a
Genes on the same
chromosome whose
alleles are so close together that they do not
assort independently
(such as a, b, and c) are
said to be genetically
linked.
Figure 12.UN05
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